This invention relates to aluminum alloys and more particularly, it relates to a control process for producing aluminum alloy sheet product having controlled recrystallization.
In many instances, continuous casting of molten aluminum into slab utilizing twin belt or twin roll casters is favored over DC casting because the twin belt or twin roll casting can result in substantial energy savings and total conversion cost savings compared to the DC cast method. In the twin belt or twin roll process, molten metal is continuously introduced to an advancing mold and a slab is produced which may be continuously formed into a sheet product which is collected or wound into a coil. However, the continuous process is not without problems. The texture and grain structure formed during processing of the molten aluminum into a sheet product determines mechanical anisotropy which strongly influences formability of the final product. Conventionally, texture and grain structure are determined after a coil of the sheet product is produced. However, if the texture and grain structure are unsuitable for final formability purposes, e.g., low formability and unsuitable earing, then the coil has to be scrapped or reprocessed, greatly adding to the cost of sheet product having acceptable formability. Thus, there is a great need for a process for continuous casting and rolling aluminum into a sheet product which avoids these problems.
The continuous casting of molten aluminum and rolling slab produced therefrom into a sheet product is disclosed in various patents. For example, U.S. Pat. No. 5,976,279 discloses a process for continuously casting aluminum alloys and improved aluminum alloy compositions. The process includes the steps of continuously annealing the cold rolled strip in an intermediate anneal using an induction heater and/or continuously annealing the hot rolled strip in an induction heater. The alloy composition has mechanical properties that can be varied selectively by varying the time and temperature of a stabilizing anneal.
U.S. Pat. No. 6,264,765 discloses a method and apparatus for casting, hot rolling and annealing non-heat treatment aluminum alloys. The method and apparatus comprises continuous casting, hot rolling and in-line inductively heating the aluminum sheet to obtain the mechanical properties within the specification tolerance of the hot rolled product.
U.S. Pat. No. 5,985,058 discloses a process for continuously casting aluminum alloys and improved aluminum alloy compositions. The process includes the step of heating the cast strip before, during or after hot rolling to a temperature in excess of the output temperature of the cast strip from the chill blocks. The alloy composition has a relatively low magnesium content yet possesses superior strength properties.
U.S. Pat. No. 5,993,573 discloses a process for continuously casting aluminum alloys and improved aluminum alloy compositions. The process includes the steps of (a) heating the cast strip before, during or after hot rolling to a temperature in excess of the output temperature of the cast strip from the chill blocks and (b) stabilization or back annealing in an induction heater of cold rolled strip produced from the cast strip.
U.S. Pat. No. 5,833,775 discloses an aluminum alloy sheet and a method for producing an aluminum alloy sheet. The aluminum alloy sheet is useful for forming into drawn and ironed container bodies. The sheet preferably has an after-bake yield strength of at least about 37 ksi and an elongation of at least about 2 percent. Preferably the sheet also has earing of less than about 2 percent.
U.S. Pat. No. 6,044,895 discloses a continuous casting and rolling system for steel strips which includes a vertically working two-roll casting device, a first device for adding molten steel to the casting device, a second device for guiding a cast strip produced by the casting device into a horizontal position, a horizontally working rolling mill for working the cast strip, and a reel device receiving the strip worked in the horizontally working rolling mill. Each of the casting device, the first device, the second device, the horizontally working rolling mill and the reel device are controlled by respective individual closed-loop control systems.
U.S. Pat. No. 5,839,500 discloses a method which enhances the quality of cast metal being cast in a continuous casting process. In particular, the method combines temperature and quality control sensing to achieve closed-loop control of the cooling of molten metal in a continuous caster.
U.S. Pat. No. 4,066,114 discloses the continuous casting of steel supervised and controlled by measuring total heat flow and the ratio of upper to lower heat flow into the mold and causing these two valves directly or indirectly to be represented in a two dimensional field. The resulting operating point must remain within an empirically predetermined range for safe operation without skin rupture.
U.S. Pat. No. 4,306,610 discloses a method for controlling the casting rate in the continuous casting of liquid metals by monitoring the casting temperature downstream from the continuous casting mold and opening or closing the bottom-pour nozzles on the hot metal vessels when the casting temperature at such point deviates from a preselected temperature range. The method includes switching of the control strand in multiple strand casters whenever the control strand has some difficulty.
U.S. Pat. No. 4,721,154 discloses that during the melt spinning process for producing metal foils having an amorphous structure, molten metal is cast through a slot-like nozzle onto a surface or wall which is rapidly moved past the nozzle. A particularly rapid quenching and cooling rate of the solidifying melt is achieved by providing cooling support elements which are supplied with a cooling pressure medium on one side of the moved surface or wall and which aide is located opposite to or remote from the nozzle. The surface or wall is constructed as a thin-walled cylindrical shell or tube which is elastically deformable to some extent. In its shell interior, there are provided a number of rows of cooling support elements which may be controlled by thickness sensors and temperature profile sensors. There is thus rendered possible, the continuous production of amorphous metal foils.
U.S. Pat. No. 5,069,267 discloses an automatic foundry plant of the kind in which two or several sideways switchable switching conveyors are placed between the pouring station and the extraction station in order to obtain the requisite cooling of the poured molds before these arrive to the extraction station and at the same time to limit the length of the plant, the new feature consists in automatic equipment, which controls the crosswise movements of the switching conveyors to take place at times when the risk is as small as possible of damaging molds situated in the transition region between the mold conveyor and the switching conveyor in question.
U.S. Pat. No. 5,454,417 discloses a method for casting steels on arcuate continuous casting installations, a steel melt being passed through a water-cooled chill mold from which a steel product emerges which has solidified at the surface and is cooled by the action of coolants for the further solidification, deflected circularly over supporting rollers, and subsequently bent from the arc by means of multi-point bending equipment back into the horizontal. The force for straightening the arc-shaped steel product is calculated from the high-temperature properties of the steel determined from high-temperature tensile tests and from the temperature profiles of the steel product calculated from surface temperatures. By a theoretical-actual comparison of values of the calculated straightening force and the actually determined straightening force, the casting conditions and the cooling are controlled so that the steel product is treated as gently as possible during the straightening, whereby surface fissures can be largely excluded.
U.S. Pat. No. 5,673,746 discloses a liquid metal/solid metal interface detecting device which comprises in general a radiation source for generating gamma radiation, which is directed to pass through a strand extruded from a continuous casting mold. A detector detects the gamma radiation passing through the partially solidified strand to determine a spatial profile for a liquid metal/solid metal interface by relying on the different gamma radiation attenuation characteristics of the solid metal and the liquid metal. Preferably, the gamma radiation is at energies of greater than one million electron volts. In some embodiments, a movable support carries the radiation source and the detector and moves the radiation source and detector along and around the ingot enabling generation of a three-dimensional profile of the liquid metal/solid metal interface by utilizing tomographic imaging techniques. Alternatively, solidification at a single region is determined and this information is used to control the formation of the strand in process controller implementations. Surface temperature detectors can also be used to provide more information about the solidification.
U.S. Pat. No. 5,697,423 discloses a continuous block caster having a temperature sensor in each of the chilling blocks. The sensor monitor and control temperatures of the chilling blocks in the casting direction (the xe2x80x9cx-directionxe2x80x9d) and the direction transverse to the casting direction (the xe2x80x9cy-directionxe2x80x9d).
In spite of these disclosures, there is a great need for a control method for continuously casting slab to be rolled into aluminum sheet, the method adapted to be adjusted on a continuous basis to provide aluminum sheet having high levels of formability and low or optimum levels of earing, for example, which avoids the need for reprocessing or scrapping coils of sheet product.
The term xe2x80x9cformabilityxe2x80x9d when used herein is used to describe the ease with which a sheet of metal can be shaped through plastic deformation. Formability of a metal can be evaluated by measuring strength, ductility, and the amount of deformation to cause failure.
Earing occurs with the peaks of the projections resulting from directional differences in the plastic working properties of rolled metal located at 45 degrees and/or 0 or 90 degrees to the rolling direction. Degree of earing is the difference between average height at the peaks and average height at the valleys, divided by the average cup height, multiplied by 100 and expressed in percent. High earing is a problem, for instance in beverage container making operations. It can cause jamming of can-making machinery and result in a substantial amount of scrap upon trimming the container. Thus, preferably earing should be less than 2% for can body sheet.
The term xe2x80x9caluminumxe2x80x9d when used herein is meant to include aluminum and its alloys.
It is an object of the invention to provide an improved process including continuous casting and rolling to continuously produce aluminum sheet product having consistent levels of formability.
It is another object of the invention to provide a process including continuously casting a slab and rolling said slab into a sheet product wherein the process is controlled using texture and grain structure to produce consistent or optimum levels of formability and earing.
It is still another object of the invention to provide a system employing continuous casting of molten aluminum into slab and rolling the slab into sheet product having consistently high levels of formability and low or optimum levels of earing by monitoring sheet texture and grain structure to control heat input to the system.
And yet it is another object of the invention to provide an improved process for producing aluminum sheet product employing a continuous caster to produce slab, continuously rolling said slab to produce a sheet product and continuously annealing the sheet product, the texture and grain structure of the sheet product monitored to maintain or adjust the temperature of annealing.
Still it is another object of the invention to provide a continuous caster, rolling and annealing system to produce high formability aluminum sheet using a controller to compare the degree and/or type of recrystallization of the aluminum sheet on a continuous basis to previous recrystallization measurements and control heat to the system such as the annealing temperature to maintain the desired degree and type of recrystallization.
It is another object of the invention to provide aluminum alloys such as AA3XXX and AA5XXX series alloys in sheet stock having high levels of formability and optimum levels of earing by monitoring texture and grain structure on a continuous basis for adjusting the process to improve formability and earing.
These and other objects will become apparent from a reading of the specification and claims appended hereto.
In accordance with these objects, there is provided a process for producing aluminum sheet product having a controlled recrystallization using a continuous caster to cast molten aluminum into a slab comprising the steps of providing a source of molten aluminum and continuously casting the molten aluminum into a slab; continuously rolling the slab into a sheet product and continuously annealing the sheet product in a controlled temperature range; measuring the degree and type of recrystallization of the sheet product to provide a recrystallization related signal on a continuous basis; and relaying the signal to a controller. In the controller, comparing the signal to previous signals reflecting the degree and type of recrystallization of the sheet product to provide a comparison; and in response to the comparison, maintaining or changing heat input to the process, e.g., the annealing step, upwardly or downwardly to increase or decrease the temperature to produce aluminum sheet having the desired recrystallization. Alternatively, the temperature of hot rolling may be increased or decreased to provide a sheet product having the required degree and type of recrystallization.